Einen Button zum Bild Upload Screen hinzufügen

add_action( 'pre-plupload-upload-ui', 'wpse_20160202_pre_plupload_upload_ui' );

    add_action( 'post-upload-ui', 'wpse_20160202_post_upload_ui' );

    function wpse_20160202_pre_plupload_upload_ui()

    {

        # see https://core.trac.wordpress.org/browser/tags/4.4.1/src/wp-admin/includes/media.php#L1902

        print '<button onclick="javascript:alert(\'Upload From Dropbox\');" id="db-upload-btn" class="button media-button button-primary button-large" style="margin-bottom:10px;">Upload From Dropbox</button>';

    }

    function wpse_20160202_post_upload_ui()

    {

        # see wp-includes/media-template.php

        print '<button onclick="javascript:alert(\'Another Upload From Dropbox\');" id="db-upload-btn" class="button media-button button-primary button-large" style="margin-bottom:10px;">Another Upload From Dropbox</button>';

    }